Separate brushes?

[Silent Invader]

Same brushes but a different water pot and I clean the brush with white spirit after.

[Orctrader]

Separate brushes.

After I've painted the figure's base I then do the metallics.  Then I reapply the black undercoat wherever the metallic paint has strayed over the line.  Nothing worse than seeing a tiny glint of a rogue metallic flake on a non-metallic part of the figure.

Once metallics are completely dry I move on to the rest of the figure.  Rarely during the same painting session.

[OSHIROmodels]

Separate water for me (if I remember  )

cheers

James

[Hammers]

I don't bother with any of this malarky, I paint metallics with the same brush I paint everything else with, use the same water and intersperse my metallic applications with normal paint willy-nilly.
I haven't noticed any problems.
Still, quantity over quality is my watch word

And what kind of brush would that be? A toilet brush? 

[former user]

toilet brushes are handy for applying strokes of dirt